1. Field of the Invention
This invention relates to a high-precision cog system measuring instrument which measures the shapes of precise mechanical parts, and which is suitable for evaluating shapes in a cylindrical coordinate system or a polar coordinate system, such as a cog.
2. Description of the Related Art
As shown in FIGS. 4A and 4B, conventional cog measuring instruments used in measuring, for instance, the tooth shape, pitch, and tooth line, comprises a rotating table, attached to the horizontal arm of a three-dimensional measuring instrument, or a modification thereof (Cf. “Coordinate Measurement Part 2: Evaluation of Performance of Three-dimensional Coordinate measuring instrument” published by the Japanese Standards Association. JIS B 7440-2: 1997 (ISO 10360-2: 1994)).
In the device shown in FIG. 4A, a table TX moves in the X direction and is provided on a base BS; a table TZ moves in the Z direction and is provided next to the table TX; a table TY moves in the Y direction and is provided next to the table TZ. A supporting column CL is provided at the position touched by a measuring element P at the tip of the Y-direction moving table TY, and supports the vertical axis of the object to be measured W while enabling it to rotate.
In the device shown in FIG. 4B, the object to be measured W is placed on the X-direction moving table, and the measuring element P is attached to the Z-direction moving table TZ on the Y-direction moving table, which is provided on the base BS.
Each of the measuring instruments shown in FIGS. 4A and 4B comprises a part which supports the object to be measured W, and a mechanism moving in three or two axes for supporting the measuring element P.
According to this constitution, in the measuring instruments shown in FIGS. 4A and 4B, the part which supports the object to be measured W, and the mechanism moving in three or two axes which supports the measuring element P, apply separate loads.
FIGS. 5A and 5B show how the part which supports the object to be measured W, and the mechanism moving in three or two axes which supports the measuring element P, become deformed as a consequence of these loads.
Therefore, the base BS becomes warped by these two parts, making it difficult to match the origin and axial directions of the absolute coordinate system which the object to be measured W belongs to with the origin and axial directions of the coordinate system taking the measuring element P as a reference. Consequently, there are inevitable errors in the measurement of the cog, making it impossible to increase precision.